1. Field of the Invention
The present invention generally relates to signal filtering, and more particularly to a low-pass filter.
2. Description of Related Art
Conventionally, when a wireless network product is working at high power, harmonic components of high frequencies are generated by the nonlinear properties of the active components, causing electromagnetic interference (EMI).
To solve this problem, the manufacturers of such wireless network products often use a filter to suppress the noise generated by the harmonic components. Features of an ideal filter are zero signal attenuation within a pass band, becoming infinite within a stop band, and transition as sharp as possible from the pass band to the stop band, providing the shortest possible distance between a transmission zero point and the stop band. In addition, increased transmission zero points improve performance of the filter in suppression of harmonic noise.
Referring to FIG. 4, a conventional low-pass filter 40 is shown. The low-pass filter 40 includes an input portion 400, an output portion 420 aligned with the input portion 400, a high impedance transmission portion 440 electrically connected to the input 400 and the output 420, a rectangular first low impedance transmission portion 460 electrically connected to the high impedance transmission portion 440, and a rectangular second low impedance transmission portion 480 parallel to the first low impedance transmission portion 460 and electrically connected to the high impedance transmission portion 440. The input portion 400 inputs the electromagnetic signal. The output portion 420 outputs the electromagnetic signal. The input portion 400 and the output portion 1420 have impedance values of approximately 50 ohms (Ω), respectively. The first low impedance transmission portion 460 and the second low impedance transmission portion 480 have the same length and width. An overall length of the low-pass filter 40 is 8.69 millimeters (mm), and an overall width of the low-pass filter 40 is 3.53 mm. An area of the low-pass filter 40 is 30.67 mm2.
FIG. 5 is a diagram showing a relationship between amplitude of insertion or return loss and frequency of an electromagnetic signal traveling through the low-pass filter 40. As shown in FIG. 5, only one transmission zero point is generated, such that the low-pass filter 40 is not effective in the suppression of harmonic noise.
Therefore, a heretofore unaddressed need exists in the industry to overcome the described limitations.